Conventionally, thermosetting resins such as phenol resin, melamine resin, epoxy resin, unsaturated polyester resin and bismaleimide resin are widely used in a wide variety of industries because of having thermosetting property and its excellent water resistance, chemical resistance, heat resistance, mechanical strength and reliability.
However, there are drawbacks that phenol resins and melamine resins generate volatile by-products upon curing and epoxy resins and unsaturated polyester resins have poor flame retardancy and bismaleimide resins are very expensive.
In order to solve these drawbacks, polybenzoxazine which is thermally cured without involving the generation of volatile components has been studied.
Benzoxazine has a complex structure of benzene ring and oxazine ring. In the case of thermosetting resins with benzoxazine ring in molecular structure, the oxazine ring is opened by heating and the polymerization proceeds without the formation of by-products. The benzoxazine-containing polybenzoxazines are attracting attention as a thermosetting resin for use in sealing materials, impregnation, laminates, adhesives, paints, coating materials, friction materials, FRP and molding materials.
These polybenzoxazines are known as cured polymers with well-balanced mechanical, electrical and chemical properties including high glass transition temperature (Tg), low dielectric characteristics, high tensile strength, low thermal expansion coefficient, excellent stretchability and low hygroscopicity, but have a disadvantage that they are cured at relatively high temperatures or cured slowly.
The use of various curing agents such as phenol (JP 2000-178332A), amine (JP 2000-86863A), and phosphine (JP 2003-82099A) for lowering the curing temperature of polybenzoxazine has been reported, and also alternative curing agents based on metal-ligand complexes have been reported. WO2008/034814 A2 discloses several modified acetylacetonate metal complexes as catalysts/curing agents for low temperature curing of benzoxazine-containing compositions.
However, some of the curing agents mentioned above are highly reactive and may lead to a partial polymerization of polybenzoxazine even at temperatures of 25° C. or less, and additionally, some of the curing agents mentioned above may negatively contribute to the thermal stability of polybenzoxazine or to the thermal stability of the curing reaction product of polybenzoxazine. As a result, there is a problem that undesired weight loss is caused, in particular during polymerization/curing reaction of polybenzoxazine.
Therefore, a novel mixture of polybenzoxazines is required which is rapidly cured at low temperature without the use of a curing agent and exhibits high thermal stability and low dielectric characteristics while maintaining excellent physical properties of polybenzoxazine.